1. Field of the Invention
This application relates to the field of computer data storage and more particularly to the field of configuring a cache in a computer data storage system having multiple processors accessing the cache.
2. Description of Related Art
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units, disk drives, and disk interface units. Such storage devices are provided, for example, by EMC Corporation of Hopkington, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlizzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may not correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data stored therein.
Performance of a storage system may be improved by using a cache. In the case of a disk drive system, the cache may be implemented using a block of semiconductor memory that has a relatively lower data access time than the disk drive. Data that is accessed is advantageously moved from the disk drives to the cache so that the second and subsequent accesses to the data may be made to the cache rather than to the disk drives. Data that has not been accessed recently may be removed from the cache to make room for new data. Often such cache accesses are transparent to the host system requesting the data.
One technique for implementing a cache is to store the data in blocks and link each of the blocks together in a doubly linked ring list referred to herein as a xe2x80x9clogical ring unitxe2x80x9d (LRU). Each block of the LRU represents a block of data from a logical disk unit. The blocks are placed in the doubly linked ring list in the order in which they are retrieved from the disk. A pointer may point to the block that was most recently added to the list. Thus, when a new block is to be added to the cache, the structure of the LRU, in combination with the head pointer, may be used to determine the oldest block in the LRU that is to be removed to make room for the new block.
A drawback with the LRU mechanism is that only one process may access and manipulate the ring list at a time since the complexity of the doubly linked ring structure makes it difficult to allow more than one process to manipulate to the data structure at any time. One way to enforce this one-at-a-time access is to use a software lock, which is a conventional semaphore-like mechanism that allows a process exclusive access to the LRU. However, when multiple processors need to use the cache, then the exclusive LRU access policy may become a bottleneck. In addition, in some instances, it may be desirable to provide a mechanism for adjusting cache services provided to the host processor systems coupled to the storage device so that some of the host processors may receive better cache performance than other ones of the host processors.
According to the present invention, storing data in a cache memory of a storage device includes providing access to a first segment of the cache memory on behalf of a first group of external host systems coupled to the storage device and providing access to a second segment of the cache memory on behalf of a second group of external host systems coupled to the storage device, where at least a portion of the second segment of the cache memory is not part of the first segment of the cache memory. In some embodiments, no portion of the second segment of the cache memory is part of the first segment.
Storing data in a cache memory of a storage device may also include providing a first data structure in the first segment of the cache memory and providing a second data structure in the second segment of the cache memory, where accessing the first segment includes accessing the first data structure and accessing the second segment includes accessing the second data structure. The data structures may be doubly linked ring lists of blocks of data. Each block of data may correspond to a track on a disk drive.
Storing data in a cache memory may also include apportioning the cache memory into slots and mapping each of the slots to at least one of the first and second segments of the cache memory. The slots may be mapped to the segments using a formula or a table. The groups may be mapped to particular ones of the segments using a table. The table may include group identifiers and corresponding masks. The masks may be binary values that have a xe2x80x9conexe2x80x9d bit in an Nth bit position to indicate that a group is assigned to an Nth segment. Storing data in a cache memory may include, in response to a request for a block of cache memory, determining availability of a block of cache memory for a group mapped to the cache memory. In response to no blocks of cache memory for the group being available, a block of cache memory corresponding to another group may be provided. The block of cache memory that is provided may be at least one of: a next available block, a block corresponding to a group having a greatest number of blocks assigned thereto, a block corresponding to a group having a greatest number of available blocks, and a block corresponding to a group having a greatest percentage of available blocks.
According further to the present invention, a cache memory of a storage device includes a first segment of the cache memory that is accessed on behalf of a first group of external host systems coupled to the storage device and a second segment of the cache memory that is accessed on behalf of a second group of external host systems coupled to the storage device, where at least a portion of the second segment of the cache memory is not part of the first segment of the cache memory. In some embodiments, no portion of the second segment of the cache memory is part of the first segment.
The cache memory may also include a first data structure in the first segment of the cache memory and a second data structure in the second segment of the cache memory, where accessing the first segment includes accessing the first data structure and accessing the second segment includes accessing the second data structure. The data structures may be doubly linked ring lists of blocks of data. Each block of data may correspond to a track on a disk drive. The cache memory may also include a plurality of slots, each corresponding to a portion of the cache memory, where each of the slots is mapped to at least one of the first and second segments of the cache memory. The slots may be mapped to the segments using a formula or a table. The groups may be mapped to particular ones of the segments using a table. The table may include group identifiers and corresponding masks. The masks may be binary values that have a xe2x80x9conexe2x80x9d bit in an Nth bit position to indicate that a group is assigned to an Nth segment.
According further to the present invention, a storage device includes a plurality of disk drives, a plurality of disk interface units, each being coupled to one of the disk drives, a bus that interconnects the disk interface units and a cache memory, coupled to the bus, the cache memory having a first segment that is accessed on behalf of a first group of external host systems coupled to the storage device and a second segment that is accessed on behalf of a second group of external host systems coupled to the storage device, where at least a portion of the second segment of the cache memory is not part of the first segment of the cache memory.
According further to the present invention, storing data in a cache memory of a storage device includes providing access to a first segment of the cache memory on behalf of a first group of external host systems coupled to the storage device, providing access to a second segment of the cache memory on behalf of a second group of external host systems coupled to the storage device where at least a portion of the second segment of the cache memory is not part of the first segment of the cache memory, in response to a request for a block of cache memory by an external host system of the first group, determining availability of a block of cache memory in the first segment of the cache memory, and in response to no blocks of cache memory in the first segment being available, providing a block of cache memory from the second segment for use by the external host system of the first group. Storing data in a cache memory of a storage device may also include providing a first data structure in the first segment of the cache memory and providing a second data structure in the second segment of the cache memory, where accessing the first segment includes accessing the first data structure and accessing the second segment includes accessing the second data structure. The data structures may be doubly linked ring lists of blocks of data. Each block of data may correspond to a track on a disk drive.